(a) Field of the Invention
The invention relates to an element of a thermoplastic of the polyester or polyamide type, preferably of polyethylene terephthalate, the element consisting of an edge part which surrounds a body in an arrangement in which the latter is sunk relative to the edge part. The element is formed from a blank of mainly amorphous material or from a material having a crystallinity of less than 10%, the blank being for example, a flat plate, a blank shell or the like.
(b) Prior Art
In the manufacture of products from thermoplastics, the starting material is in most cases a virtually flat blank. Either an end product is formed here substantially in one deformation step, or a premoulding is formed for later reshaping to give the end product. The shaping of the blank is effected, according to methods known at present, either by the blow-moulding process or by the thermo-forming process. In the blow-moulding process, thick sections are as a rule obtained in the bottom. In the thermoforming process, either so-called negative thermoforming or socalled positive thermoforming is used. In the negative thermoforming process, a thin bottom is obtained, whilst a thick bottom is obtained in the positive thermoforming process.
In negative thermoforming, a warm sheet or a warm film is placed over cavities, after which the material of the film or the sheet is pressed and sucked into the cavities by external pressure and internal reduced pressure. This has the result that the material is stretched and becomes thin, when it is sucked into the particular cavities. If the cavity is a cup, a thin stretched bottom and a wall thickness increasing in the direction of the edge of the cup are obtained.
In positive thermoforming the cup mould forms a projecting body and the material of the film or sheet is pressed and sucked over this projecting body. This has the result that the material on the upper part of the projecting body, that is to say the bottom of the cup, remains thick and essentially unstretched, whilst the thickness of the material decreases towards the edge of the cup.
To obtain an adequate material thickness in the bottom part of the cup in negative thermoforming, a sufficient thickness in the starting material must be chosen. To obtain an adequate thickness in the edge zone of the cup by positive thermoforming, which is necessary for stability of the cup, a sufficient thickness of starting material must likewise be chosen. In negative thermoforming, the material zones between the shaped cups remain uninfluenced and are subsequently severed, after the manufacture of the actual cups. In positive thermoforming, the material between the cups is drawn into recesses and severed from the cups formed. In positive thermoforming, cup bottoms are thus obtained which have substantially the same thickness as the starting material. Both forming processes require an unnecessarily high consumption of material, which is of economic importance in the mass production of articles.